Conclusively (4,3,3,3)
I think it could be either:
once and for all
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">conclusively</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for once and for all that I've seen before include "Finally" , "For the last time" , "how might an inclusively unique show be performed?" , "unique public statement?" , "In a conclusive way" .)